Selective suppression of all classical MHC genes has a physiological basis in pregnancy. The suppression occurs only in special placental endothelial cells, the trophoblasts. These cells separate the maternal and fetal circulations and hence their immune systems. Accordingly, a highly conserved and redundant mechanism preserves phylogeny by creating immunologic privilege in pregnancy. The efficiency of the system is complete. The fetus is 50% or, with in vitro fertilization, can be 100% foreign DNA to the mother, yet no rejection occurs. In veterinary experiments, a zebra ovum implanted in a horse resulted in the birth of a normal zebra, yet there is a species difference. The ability to efficiently abrogate the interaction of donor MHC antigens and their presented peptides with recipient lymphocytes would greatly reduce or eliminate the need of immunosuppression and, along with alpha-gal knockouts (to eliminate antibody-mediated hyperacute rejection), would be a critical step in the production of animals suitable for xenografts. Trophoblasts contain dominant negative transacting factors that suppress all classical MHC genes. The mechanism of class I suppression is at present unknown. However, a 481 nt cloned trophoblast noncoding RNA (TncRNA) suppresses CIITA expression by 85-90%. We believe TncRNA is an incompletely processed miRNA that acts by partial complimentary binding to CIITA promoters III and IV possibly between transcription factors binding sites on promoter IV. Proposed are experiments to inhibit TncRNA processing to miRNA by siRNA Dicer depletion and test its effect on class II expression, define the TncRNA domain on or near pIll and pIV by site directed mutagenesis, clone miRNAs from HeLa cells transduced with TncRNA and Jar trophoblast cells and determine the role of methylation in CIITA promoter suppression and finally, to identify processed miRNAs by primer extension analysis. Over 87,000 individuals are awaiting organ transplants in the U.S. In 2004, 42,816 new registrants were added, while only 24,817 organs were procured. Long waiting times and ethical dilemmas regarding organ allocation to patients with end stage organ disease is now a pressing medical dilemma. [unreadable] [unreadable] [unreadable]